Dragline excavators represent a capital expenditure of hundreds of millions of dollars with operational overheads currently around US$6,000 per hour. In order to maximize operational efficiency and return on investment, this necessitates continuous operation of a dragline apparatus 24 hours a day, 7 days a week. Apart from routine shut-downs for maintenance requirements, any reduction in operational efficiency can represent substantial annual productivity losses.
Generally speaking, most draglines are a compromise between such factors as boom length, bucket and rigging mass and bucket payload capacity. Operational efficiencies of a dragline bucket can be measured according to a number of parameters including drag energy (or specific drag energy) and total sum load of the bucket, rigging and payload where:
DRAG ENERGY=a measure of the energy required to fill a bucket of given capacity. Factors affecting drag energy include the extent of frictional engagement between internal and external bucket surfaces and earth masses within and without the bucket respectively, tooth/cutting edge configurations and the dead mass of the bucket/rigging combination. SPECIFIC DRAG ENERGY=the drag energy expended per kg of payload excavated.
TOTAL SUM LOAD (TSL)=the sum of the masses of the bucket rigging and payload.
Since the early 1900's, there have been many modifications to bucket designs and rigging configurations in an endeavour to achieve greater excavation efficiencies in terms of energy consumption and excavation rates. During the last century, bucket capacities have increased from about 20 tonnes to over 100 tonnes.
Excavator bucket designs generally are of an arched or archless design with some excavator operators preferring an arched design at the expense of reduced payload to obtain a more robust bucket with lower maintenance requirements. Generally the mass of an archless bucket and rigging is less than that of an arched bucket and associated rigging largely due to the exclusion of the arch over the front of the bucket. It is argued in some quarters that increased productivity offsets any increases in maintenance of a less robust archless bucket but, at the end of the day, the decision as to which bucket is employed is often predicated on the type of earthen material to be excavated with the archless bucket being used with softer, less aggressive, easily penetrated earth types.
Examples of archless buckets are described in U.S. Pat. Nos. 2,096,773; 2,334,460; 3,247,606; 3,247,607, 5,400,530 and 5,832,638, whereas examples of arched buckets are described in U.S. Pat. Nos. 3,597,865; 4,791,738; 4,944,102; 5,140,761; 5,307,571; 5,343,641; 5,343,702; 5,428,909; 5,575,092 and 6,705,031.
The archless buckets referred to above are generally of a low mass and are fabricated from steel plate components with generally parallel side walls, a rearwardly inclined rear wall and, with the exception of U.S. Pat. Nos. 3,247,606 and 3,247,607 which have side walls perpendicular to a floor, all others describe outwardly and upwardly inclined side walls. The geometry of these buckets was claimed to increase bucket payload and to provide less frictional resistance between the earth mass and the bucket during loading.
The arched buckets described above generally comprise generally slab sided structures with side walls perpendicular to a floor and an arcuate transition between the floor and a rear wall which may incline outwardly or inwardly towards the top thereof. The arched buckets generally have a more robust construction than the archless buckets described above, and generally are fabricated from sheet steel components and cast components such as the bucket lip, cheek plates, the arch member and/or arch mountings. Reinforcing members such as trunnion mounting plates and a cap rail formed along the upper edges of the side and rear walls were generally fabricated from sheet steel.
The above-mentioned prior art excavator buckets are illustrative of on-going endeavours for over a century to produce more efficient buckets while over that same period accommodating demands for buckets with greater load capacity. In the many, many patents granted for improvements in excavator buckets over the last century or so, most of those inventions dealt with single incremental improvements which may have improved one aspect of the performance of the bucket but often at the expense of one or more other functional or structural aspects of the bucket whereby the overall or net benefits represented only a marginal improvement.
Some of the shortcomings of the prior art excavator buckets were addressed in U.S. Pat. No. 6,834,449 to the same assignee. This patent described a light-weight high capacity archless bucket which exhibited a payload increase of about 10% over competitors' conventional buckets along with a reduction in drag energy of about 30% of that of a conventional bucket while at the same time reducing bucket fill time by 20%. This bucket was robust in nature with a cast front lip, cheek plates and junction members between side and rear walls to provide a smooth arcuate transition therebetween to reduce frictional engagement with a mass of earth during filling and emptying of the bucket. The bucket had a wide, relatively shallow configuration when compared to conventional excavator buckets at that time and was distinguished by a rear wall being higher than that adjacent side wall portions with a steep arcuate taper between the floor and the top of the rear wall.
The bucket of U.S. Pat. No. 6,834,449 had a side wall height:lip width ratio of about 1:4 compared with conventional prior art buckets having a ratio of about 1:1.5 to 1:2.
In plan view, the side walls converged toward the rear wall such that the rear portion of the bucket was about 80% of the effective width of the opening between opposed cheek plates.
While generally effective for its intended purpose, the excavator bucket described in U.S. Pat. No. 6,834,449 was suited more to lighter, softer earth types rather than harder rock filled earth types found in certain regions.
Accordingly, it is an aim of the present invention to overcome or alleviate at least some of the shortcomings of prior art excavator buckets and otherwise to provide a robust heavy duty excavator bucket which still exhibits the improved operational efficiencies of the light-weight buckets described in U.S. Pat. No. 6,834,449.